When comparing it with other import/export scripts it looks like the registering process requires classes to be defined, which have register / unregister methods included.
The old method probably allowed this to done with simple functions.

So the challenge would be to structure the register / unregister in classes as it is done for the other import and export scripts.

nope, it doesn't work. did you just update the version? because it is not enough. previously I went on and also updated the class registration, it installed fine but it didn't show up in my export menu.

I am wondering also if there is any compatibility issue between blender files versions. I think there is not actually... so one option could be to model and test render in 2.8 and then just export through 2.79.

How classes are registered and named is also changing. Scripts and blend files are not backward compatible.

As has been noted, this is Beta software and any changes made to scripts may or may not work in the next release. I'd stick with Blender 2.79 for serious Orbiter development until things are locked down a bit tighter with 2.80.

BTW, I do like the direction Blender is taking. It should soften the learning curve for many coming from other applications.

Thanks for the feedback ... and my apologies, guys. It was a late Saturday night thing while I was tinkering with a new Raspberry Pi, so I made the quick minimum change and then pulled a Microsoft and let the community do the testing. I abase myself.

I need it working too so I will continue working on the rest of it ... and then test it first. It would be better if Vlad came back to play as the original author always knows his software best. Please feel free to add in your own efforts, too.

I haven't tried it yet, but I think there is still the option of saving a Blender model as .obj and then using the obj to msh converter.

While I've been researching the required changes to Vlad's Blender i/o addon, I noticed that Blender 2.8 can output 3ds flies. Has anyone tried this yet and then passed the output through the max to msh converter successfully?

I played around with Blender recently. The io_orbiter_msh works with version 2.79.

Unfortunately not with the upcoming version 2.80! When trying to activate it, an error message is presented: "This script has not been updated to version 2.80. Ignoring."
This seem related to how the scripts are registered in Blender 2.80.

I don't know if Vlad is still around and could update his script, please?
The new Blender version 2.80 is a major improvement on the (somewhat cluttered) interface of Blender, much more friendly to use.

It looks like Python API has changed in Blender 2.8.
Also the Blender render is replaced with Eevee, the script should export Eevee materials into a .msh file.
BTW, I didn't launch Orbiter for some years. Are there any improvements in Orbiter rendering engine that require some additional material data?
I will investigate the changes if I have free time.